1. Field of the Invention
The present invention generally relates to a drying method for various coated layers and a drying device therefor. Particularly, the present invention relates to a drying method and a drying device for various coated layers, which method utilizes specific spectrum infrared radiation, such as near infrared radiation, which has a high transmissivity relative to a coated layer on a substrate and a high absorbtivity relative to the substrate. More particularly, the present invention relates to a drying method and a drying device for various coated layers, which method utilizes a combination of near infrared radiation and the blowing of hot air.
2. Description of Prior Art
Conventionally, various drying methods employing a hot air furnace, a far infrared radiation furnace and the like have been well known and commonly used to dry a coated material on a substrate such as a metal plate and the like. The substrate provided with the coated material to be dried is referred to as a work, and the substrate per se is referred to as a method material in this specification. The drying process and the function of these drying methods have been understood as follows.
First, a work whose mother material is coated with a paint mainly composed of resin, such as an acrylic resin, is set in a furnace. The work is subjected to a blow of hot air or far infrared radiation. The solvent of the coated material is firstly evaporated from the work surface and the surface is gradually solidified while losing flowability from the surface layer. Further the solidification of the coated layer is accelerated by heating when the heat from the hot air is transmitted to the inside of the work; i.e., the mother material. On this occasion, the solvent existing in the inside of the surface is gasified and the solvent gas pierces through the solidified surface layer to evaporate from the work surface. Thus many fine pores and pin holes are generated in the work surface. In order to prevent the work surface from generating these pores and pin holes, conventional furnaces must be controlled to slowly increase the heating temperature after the solvent is evaporated from the work in a setting room.
These conventional drying methods employing such a process, require relatively long periods to complete the drying operation because the drying temperature must be kept at a low level to avoid generating the pores and pin holes. This is a serious problem to overcome. Particularly, in a specific type furnace employing a combination of infrared radiation and a blow of hot air for the purpose of a quick drying, the surface temperature of the work remarkably tends to be higher which causes the difference of temperature between the surface and the coated layer and the interface between the coated layer and the metal substrate. This temperature difference accelerates the generation of pores and pin holes in the coated layer.
In addition to the above conventional methods, various drying methods are disclosed in Japanese Patent Application for Utility Model, Laid-Open Publication No. 1-151873, entitled "Near Infrared Radiation Stove for Liquid and/or Powder Coatings"; Japanese Patent Application for Utility Model, Laid-Open Publication No. 2-43217, entitled "Light Panels for Exclusive Use in Furnace for Banking Coating Material"; and U.S. Pat. No. 4,863,375 entitled "Baking Method for Use with Liquid or Powder Varnishing Furnace". One of these documents relates to a baking method in a near infrared radiation stove for liquid and/or powder coatings. This method utilizes the properties of near infrared radiation such as quick heating at a high temperature with a remarkable penetration to improve the baking method in the stove so that the coated substance can be quickly dried and its adhesion can be also increased. In detail, liquid type or powder in liquid type coating material is applied on the surface of a substrate and is then subjected to a melt-heating work to realize a uniform coating layer on the substrate surface. Another document relates to a drying furnace employing a near infrared radiation whose light source is provided at its rear portion with a ceramic reflector containing a heater and a drying method which uses a drying furnace in which a high temperature section and a low temperature section are sequentially formed.
On the other hand, "medium wave infrared radiator" is disclosed in "Coating Technique" special October number, pp 211 to 213, issued on Oct. 20, 1990, published by K.K. Rikoh Shuppan (Science and Technology Publishing Company Inc.). This document teaches that radiated energy impacting on a coated layer is partially absorbed by the coated layer, reflected by the layer and transmitted through the layer, respectively. The absorbed energy changes to heat energy which causes the drying of coated layer. Further, the transmitted energy causes the substrate or the mother material of the coated layer to be heated so that the coated layer is heated from the inside.
Generally, physical properties of infrared radiation are known as follows.
(1) Near infrared radiation: temperature is 2,000.degree. to 2,200.degree. C. the maximum energy peak of the wave length is generated at about 1.5 .mu.m, energy: density is high, reflected and transmitted energy are greater, rising speed is fast (1 to 2 sec), life time is short (about 5000 hours).
(2) Medium infrared radiation: temperature is 850.degree. to 900.degree. C., the maximum energy peak of the wave length is generated at about 2.5 .mu.m, energy density is medium,
absorbed energy and transmitted energy are balanced so that energy can be permitted into the inside of the coated layer, life time is long.
(3) Far infrared radiation: temperature is 500.degree. to 600.degree. C. the maximum energy peak of the wave length is generated at about 3.5 .mu.m, energy density is low, energy is remarkably absorbed by the surface of the coated layer so that the surface tends to be heated, rising speed is slow (5 to 15 min), circulation loss is great.
In order to obtain a superior coating quality by using the medium wave length infrared radiation with its maximum efficiency, the following two conditions are satisfied at the same occasion.
1. Radiated energy from an infrared radiator varies as the fourth power rised value of the absolute temperature (T) of the radiator; Eb .varies. T.sup.4. In other words, the radiated energy is increased as the temperature of the radiator rises.
2. the maximum energy peak of the wave length is positioned a little to short wave length with respect to the peak absorptivity of the coated layer.
The maximum energy peak of the wave length of infrared radiation used in industrial scene for heating such coated layers is concentrated at about 3 .mu.m without exception. Therefore, the infrared radiator having the maximum energy peak of the wave length at about 2.5 .mu.m is preferable to use for effectively drying the coated layer by a combination of the absorbed energy and the transmitted energy which can effectively and uniformly heat the coated layer from its surface and backsurface.
The relation between the temperature (T) of the infrared radiator and its maximum energy peak of the wave length generated at .lambda. m is represented by Wein's displacement law: EQU .lambda. m = 2897/T
When the maximum energy peak of the wave length is generated at .lambda. m 2.5, the above equation is rewritten as follows: EQU T = 2897/2.5 = (t + 273) EQU t = 880.degree. C.
Consequently, the maximum efficiency can be realized when the medium wave length infrared radiation is used while satisfying the above condition.
The above described conventional documents Japanese Patent Application for Utility Model, Laid-Open Publications No. 1-151873 and 2-43217, and U.S. Pat. No. 4,863,375, however do not teach any optimum conditions of the infrared radiation applied to the coated layer on a metal substrate. These conventional documents disclose use of near infrared radiation to dry coated layers and general explanation on the properties of the near infrared radiation to be used.
In the use of far and medium infrared radiation for drying coated layer, their wave range is so selected that the irradiated infrared energy is highly absorbed by the coated layer. This is for the purpose of heating from the layer surface. However, this will cause the generation of many pin holes or pores in the layer surface, and thus the period for drying the coated layer will be prolonged with keeping drying temperature at a low level to prevent the coated layer from generating pin holes or pores.
"Coating Technique Special October Number" does not teach any optimum conditions of infrared radiation according to a study on the absorptivity of the infrared radiation to the mother material and/or the cause of pin holes or pores generated in the coated layer. But this document gives the conclusion that the infrared radiator which provides the maximum energy peak of the wave length at about 2.5 .mu.m is preferable because its radiated energy can be effectively absorbed and transmitted to heat the surface and backsurface of the coated layer.
The inventor of this application found out that the coated layer, can be prevented from having pin holes or pores during drying by preferring the use of near infrared radiation whose wave range can easily be transmitted through the coated layer rather than a wave range having a high:absorptivity relative to the coated layer. It can be supposed that the infrared radiation transmitted through the coated layer directly heats the substrate surface and not the layer surface and the coated layer is gradually dried from its backsurface by the heat.
In the case of the metal substrate, its reflectivity against infrared radiation is increased as the wave length of the infrared radiation is prolonged and its absorptivity for thermal energy is increased as the wave length becomes shorter. As a result, when near infrared radiation is used for drying coated layers, it can be supposed that the near infrared radiation having a high transmissivity to the coated layer; that is, a poor absorptivity to the coated layer is preferably used to prevent the coated layer from generating pin holes.
Conventional drying systems and devices are too large to apply a small scale drying work for a partial repair coating in a general paint-coating work, or in a panel processing work of vehicle body. In a conventional manner, a partially repaired product must be set again in the furnace which is designed for drying the product in an ordinary paint-coating process. Since this furnace is always controlled for drying a whole body of the product, it requires further time to adjust control parameters such as temperature and heating time for drying the repaired portion. If this drying system is arranged in an automatic controlled manufacturing line such as an automotive vehicle assembly line, this line must be stopped while the drying system is used for drying the repaired portion.
In an automotive vehicle manufacturing line, many infrared lamps generating far and near infrared radiation are used as a heating source in a drying process. Although this type of heating source can heat only irradiated portion, the outside of the irradiated portion is kept at a low temperature. The heating energy is transmitted to the low temperature portions which are not applied with infrared radiation and face the ambient air, and thus drying temperature becomes irregular. This will cause a low producing efficiency with a low quality.